As is well known, regarding production of glass sheets, the so-called downdraw method is publicly known, in which glass sheets are produced by forming a glass ribbon formed by causing molten glass to flow downward from a forming body and the glass ribbon is cut into a predetermined dimension. Typical examples thereof include an overflow downdraw method (fusion method) and a slot downdraw method. In the former overflow downdraw method, glass sheets are produced by causing molten glass supplied into the forming body having a wedge-like shape in cross section to flow downward from the top of the forming body along both side surfaces thereof so as to be fused at a lower end portion of the forming body and to be formed into a single sheet-like shape, by causing the glass ribbon in this sheet-like shape to flow downward from the lower end portion of the forming body, and by finally cutting the hardened glass ribbon into a predetermined dimension. Meanwhile, in the latter slot downdraw method, glass sheets are produced by causing molten glass supplied into the forming body to flow downward from a slit in an oblong shape formed in a bottom portion of the forming body so as to be formed into a sheet-like shape, by causing the glass ribbon in this sheet-like shape to flow downward along a conveyance path, and then by finally cutting the hardened glass ribbon into a predetermined dimension.
In a glass sheet production installation of this type, in which the downdraw method is utilized, there are arranged, below a forming chamber in which the forming body is accommodated, an annealing chamber for annealing the glass ribbon, a cutting chamber for cutting the cooled glass ribbon into glass sheets having a predetermined dimension, and the like. In addition, cooling steps for the glass ribbon in the annealing chamber and the like have a significant influence on the product quality of the glass sheets finally produced. Thus, it is necessary to strictly control atmospheric temperatures therein. Under the circumstance, in order to prevent variation in the atmospheric temperature, the conveyance path for the glass ribbon vertically extends in a chimney shape, with the periphery thereof being surrounded (refer to JP 10-53426 A and JP 2001-31435 A, for example).
Incidentally, in recent years, as is typified by glass substrates for flat panel displays such as liquid crystal displays and the like, various glass sheets are demanded to be of a higher grade and to have higher quality under the present circumstances. In this context, glass sheets (defective glass sheets) which do not meet the demands are produced in producing steps thereof in many cases, and hence it is necessary to classify and to collect the defective glass sheets. Similarly, also in the case where some troubles occur in steps subsequent to a cutting step in the producing steps, a situation may occur where glass sheets cannot be loaded in the steps subsequent to the cutting step. In this case also, it is necessary to collect the glass sheets. Thus, in the conventional producing steps in the glass sheet production installation as described above, in the case where failures such as intolerable distortion and inclusion occur in the glass ribbon and the glass sheets obtained by cutting, or in the case where glass sheets cannot be loaded in the steps subsequent to the cutting step irrespective of presence and absence of the failures, the glass sheets (waste glass sheets or cullet) are discarded into a dedicated collection box arranged in the cutting chamber.
In this case, when flowing-down of the molten glass from the forming body is interrupted so as to stop forming of the glass ribbon, it becomes considerably difficult to restore a forming condition into a desired state. Thus, even when it becomes necessary to discard a glass ribbon to be formed, the glass ribbon is sequentially cut and the glass sheets obtained by cutting are discarded into the collection box while supply of the glass ribbon into the cutting chamber is maintained as it is.